Oil and gas operators drilling vertical or near vertical wellbores into production zones, usually lower steel production casing into the wellbore across the production zone and permanently cement it in place. The steel casing and cement are then perforated to allow hydrocarbon fluids to flow into the wellbore. These perforations are normally sufficient in length to penetrate beyond an zone of drilling induced damage to the production zone rock.
Damage to production zones can occur when the reservoir is penetrated in an "overbalanced" situation. In an "overbalanced" situation the hydrostatic pressure created by the annular column of drilling fluid exceeds the pressure in the reservoir. This is in contrast to an "underbalanced" situation where this same hydrostatic pressure is less than the reservoir pressure. In an "underbalanced" situation, reservoir fluids freely flow into the wellbore while the production zone is being penetrated.
Some of the problems associated with underbalanced drilling are outlined in a paper entitled "Underbalanced Drilling, Praises and Perils" presented by D. Brant Bennion, F. Brent Thomas, Ronald F. Beitz, and Douglas W. Bennion to the Society of Petroleum Engineers at a conference in Midland, Tex. in March of 1996 (SPE paper #35242).
Increasingly operators are drilling into production zones horizontally. Often steel casing and cement are not placed into the production zone. This is termed an "open hole" completion. It is difficult and expensive to perforate an open hole completion. It is, therefore, important to avoid damaging the production rock surrounding the wellbore while drilling through it.
Many kinds of damage mechanisms have been identified. An "aqueous phase trap", also called a "water block", occurs when water or "filtrate" (the liquid phase of the drilling fluid) enters the production zone. This filtrate fills up pore spaced normally occupied by gas or oil, reducing the permeability to the gas or oil. It is difficult or impossible to extract all of the filtrate such that the original permeability is restored. The zone is permanently damaged. Aqueous phase trapping is discussed in a paper entitled "Reduction in the Productivity of Oil and Gas Reservoirs Due to Aqueous Phase Trapping" by D. Brant Bennion, Mauro P. Camolai, Ronald F. Bietz and F. Brent Thomas; presented to the Petroleum Society of CIM in Calgary May 1993 (CIM paper #CIM93-24).